Most compressors will not
cover the full pressure range
efficiently. The lowest pressure is atmospheric, for gas to pipeline, some 3 to
5 MPa (30-50 bar) pressure is used, while reservoir reinjection of gas will
typically require 20 MPa (200 bar) and upwards, since there is no liquid in
the tubing and the full reservoir pressure must be overcome. Therefore,
compression is divided into several stages to improve maintenance and
availability.
Also due to single unit power limitations, compression is often divided in
several parallel trains. This is not the case in this example, since gas is not
exported and reinjection can be interrupted during maintenance periods.
Compressors are driven by gas turbines or electrical motors (for lower power
also reciprocating engines, steam turbines are sometimes used if thermal
energy is available). Often, several stages in the same train are driven by the
same motor or turbine.The main operating parameters for a compressor are
the flow and pressure differentials. The product defines the total loading, so
there is a ceiling set by the maximum design power. Furthermore, there is a
maximum differential pressure (Max Pd) and choke flow (Max Q), the
maximum flow that can be achieved. At lower flow, there is a minimum
pressure differential and flow before the compressor will "surge" if there is
not enough gas to operate.
If variations in flow are expected or differences between common shaft
compressors occur, the situation will be handled with recirculation. A high
flow, high pressure differential surge control valve will open to let gas from
the discharge side back into the suction side. Since this gas is heated, it will
also pass through the heat exchanger and scrubber so as not to become
overheated by circulation.
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Figure 8. Compressor state diagram
The operating characteristics are defined by the manufacturer. In the
diagram above, the blue lines mark constant speed lines. The maximum
operating limits are set by the orange line as described above. The surge
domain is the area to the left of the red surge curve.
The objective of compressor performance control is to keep the operating
point close to the optimal set point without violating the constraints by means
of control outputs, such as the speed setting. However, gas turbine speed
control response is relatively slow and even electric motors are not fast
enough, since surge response must be in the 100 ms range. Anti-surge
control will protect the compressor from going into surge by operating the
surge control valve. The basic strategy is to use distance between operating
point and surge line to control the valve with a slower response time, starting
at the surge control line. Crossing the surge trip line will cause a fast
response opening of the surge valve to protect the compressor.
Operation with recirculation wastes energy (which could result in
unnecessary emissions) and produces wear and tear, particularly on the
surge valve. Each vendor supplies several variants of compressor control
and anti-surge control to optimize performance, based on various corrective
and predictive algorithms. Some strategies include:
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• Set point adjustment: If rapid variations in load cause surge valve
action, the set point will be moved to increase the surge margin.
• Equal margin: The set point is adjusted to allow equal margin to
surge between several compressors.
• Model based control: Outside the compressor itself, the main
parameter for the surge margin is the total volume from the surge
valve to the compressor suction inlet, and the response time for the
surge valve flow. A model predictive controller could predict surge
conditions and react faster to real situations while preventing
unnecessary recirculation.
Since compressors require maintenance and are potentially expensive to
replace, several other systems are normally included:
Load management: To balance loading among several compressors in a
train and between trains, the compressor control system
often includes algorithms for load sharing, load shedding
and loading. Compressors are normally purged with inert
gas, such as nitrogen during longer shutdowns, e.g., for
maintenance. Therefore, startup and shutdown sequences
will normally include procedures to introduce and remove
the purge gas.
Vibration: Vibration is a good indicator of problems in compressors,
and accelerometers are mounted on various parts of the
equipment to be logged and analyzed by a vibration
monitoring system.
Speed governor: If the compressor is turbine driven, a dedicated speed
governor handles the fuel valves and other controls on the
turbine to maintain efficiency and control rotational speed.
For electrical motors this function is handled by a variable
speed drive.
The final function around the compressor itself is lube and seal oil handling.
Most compressors have wet seals, which are traps around shafts where oil
at high pressure prevents gas from leaking out to atmosphere or other parts
of the equipment. Oil is used for lubrication of the high speed bearings. This
oil gradually absorbs gas under pressure and may become contaminated. It
needs to be filtered and degassed. This happens in smaller reboilers, in
much the same way as for the glycol reboilers described earlier.
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